The art recognizes the interest of integrated electrical motor driver circuit integrating as functions as possible on the same semiconductor circuit and that are flexible enough to be used for various applications. For example, the same integrated motor driver circuit may be used to control a motor with or without sensor(s). The usual sensors known to the art for control of motors are hall sensors or optical encoders. Flexibility may also concern the format of the data and the amount of data that is made available to circuit(s) and/or components external to the integrated motor driver circuit: such data can be analog or digital in nature and if digital in nature, the number of bits used to encode the data might need to vary from one application to another in function of desired characteristics for the resulting motor drive control loop.
The art does recognize the advantage that there is to use the back electromotive force (BEMF) measured across the windings at the stator of a motor to derive signals that can be used to detect a stall condition, detect the position and/or speed of the rotor of a motor, determine the load conditions of a motor. In particular as disclosed in the published patent application EP 1 460 757 A1, it may be advantageous in certain applications to measure and use the BEMF at several sampling instants instead of a single BEMF measurement.
In the application note AN235/0788 Stepper Motor Driving by H. Sax from SGS-Thomson Microelectronics (a.k.a. ST), the art also recognizes the interest there is to reduce the number of connections between a first integrated circuit (a motor driver circuit) and a second integrated circuit (a control circuit or data processing circuit, digital or otherwise, in particular a micro-controller or micro-computer chip).
TRINAMIC Motion control GmbH & Co KG offers a solution in the integrated circuit TMC 249/A. As described in the data sheet of the TMC 249/A (version 2.02 dated August 12, 2005), the BEMF of a motor is measured in order to detect a stall condition of that motor. The BEMF measurement is made available as a digital, three bits indicator that can be shared with an external micro-controller through a serial interface.
Freescale Semiconductor offers yet another solution in the integrated circuit MM908E625. This integrated circuit consists of basically 2 dies in one package: an analogue chip and a digital (microcontroller) chip. As described in the data sheet of the MM908E625 (Rev. 4.0 dated September 2005), the BEMF of a motor is measured indirectly by means of detecting the existence and duration of a recirculation current. A signal called “BEMF output” is basically a single bit flag (binary output data from an analogue comparator that is integrated in the analogue die) that indicates if (yes or no) a recirculation current in the motor exists. This flag is generated each PWM cycle during the recirculation phase and is made available as an analogue output for connection to the second die (the embedded microcontroller) for further processing inside the integrated circuit MM908E625. Because the information of the “BEMF output” is contained in the time-domain (i.e. the duration of the pulse “BEMF output” indicates the duration of the existence of recirculation current and this is a measure for the amount of BEMF), the microcontroller needs to be equipped with a timer circuit that measures the duration of the pulse. This timing information can then be transformed into other data and can be exchanged with an external circuit, in particular an external micro-controller, through a serial interface using output pins distinct from the BEMF pin.
In yet other integrated circuits, those from the ST7MC family offered by SGS-Thomson Microelectronics, information related to the BEMF is offered under the form of a single bit binary signal on a first output pin of the semiconductor chip for use by external circuitry. The actual BEMF signal is available to a micro-controller integrated in the same semiconductor substrate as the motor driver circuit, thereby eliminating the need for transmission of detailed BEMF information to an external circuit.
There remains a need to improve on the art.